Did you know the Earth weighs 5,974,000,000,000,000,000,000,000 kilograms? That’s a mouthful. Maybe a better way to express this quantity is to say our dear blue marble weighs about 6,000 trillion kilotons. But an even neater way of expressing Earth’s mass is packing it into only six ronnagrams.

If you’re like me, this is the first time you’ve heard of a ‘ronna’ — and I don’t mean that certain something that locked you inside your home for a whole year.

Ronna is one of four new metric prefixes that scientists voted last week to be officially added to the International System of Units (SI), the global standard for the metric system. It’s meant to express quantities larger than one followed by 27 zeroes. Joining ronna, but also its more famous cousins like ‘kilo’, ‘mega’, and ‘giga’, is ‘quetta’, which means one followed by 30 zeroes.

The jump from ronna to quetta is the same order of magnitude as that from kilo to mega. Using the same planetary scale analogy, Jupiter — which has a mass 318 times Earth’s — has a mass of two quettagrams.

But it’s not extremely large masses that prompted the need for ronna and quetta. Neither did distance, for which we already have the parsec to describe the astronomical measurements of galaxies as an alternative to light-years.

The need for the new prefixes comes closer to home, prompted by our increasingly growing appetite for big data and secure cloud storage.

“In terms of expressing data in yottabytes, which is the highest prefix currently, we’re very close to the limit,” Richard Brown, the head of metrology at the UK’s National Physical Laboratory, told AFP.

The last time scientists had to enrich the SI was in 1991, when the ‘zetta’ and ‘yotta’ were introduced at the insistence of chemists who wanted to express vast molecular quantities. A zetta is one followed by 21 zeroes and a yotta is one followed by 24 zeroes.

But not even the stupendously large yottabyte couldn’t handle the world’s voracious appetite for data. According to Statista Digital Economy Compass, the world generated 33 zettabytes of data in 2018 alone and the overall global datasphere reached 64 zettabytes in 2020. 

The world’s data generation will only continue to increase at an exponential rate as the Internet of Things expands to every networked consumer device, from electric cars to your kitchen toaster. So it makes sense to introduce ronna and quetta, although people had unofficially used their own unsanctioned prefixes like the brontobyte and hellabyte. These actually sound cool, but there’s a method to naming SI prefixes.

Metric prefixes need to be shortened to just their first letter (think Kbyte or Mbyte), and B and H were already taken. Another convention is that larger prefixes need to end in an A while smaller ones end in an O (think mega and micro). The only letters that were not used for other units or other symbols were R and Q, which brings us to the fabled ronna and quetta, but also to their symmetrical pair meant to express extremely small things: the ronto and quecto for the smallest.

“At the bottom end, it makes sense to have a symmetrical expansion, which is useful for quantum science, particle physics—when you’re measuring really, really small things,” Brown said.

How long before ronna and quetta become, in their own turn, obsolete? No one can say for sure, but we should be good for another two or three decades. But at the rate artificial intelligence and big data are growing, you never really know. And when ronna and quetta are finally outgrown, scientists will have to face another problem: there are no more letters in the alphabet available to represent new prefixes. I wouldn’t lose sleep over it, though.